1. Field of the Invention
The invention relates generally to rotary drill bits and, more particularly, to rotary drill bits for use in drilling holes in subsurface formations.
2. Description of the Related Art
In the normal prior art construction, the gauge region of a drill bit is formed by a plurality of kickers which are spaced apart around the outer periphery of the bit body and are formed with bearing surfaces which, in use, bear against the wall of the borehole. The kickers generally form continuations of respective blades formed on the leading face of the bit and extending outwardly away from the axis of the bit towards the gauge region so as to define between the blades fluid channels leading towards the gauge region. The spaces between the kickers define junk slots with which the channels between the blades communicate. During drilling, drilling fluid pumped down the drill string to nozzles in the bit body flows outwardly along the channels, into the junk slots at the end of the channels, and passes upwardly through the junk slots into the annulus between the drill string and the wall of the borehole.
While such PDC bits have been very successful in drilling relatively soft formations, they have been less successful in drilling harder formations, including soft formations which include harder occlusions or stringers. Although good rates of penetration are possible in harder formations, the PDC cutters may suffer accelerated wear. Thus, bit life may be too short to be commercially acceptable.
Studies have suggested that the rapid wear of PCD of bits in harder formations may be due to chipping of the cutters as a result of impact leads caused by vibration of the drill bit. One of the most harmful types of vibration can be attributed to a phenomenon called "bit whirl," in which the drill bit begins to precess around the hole in the opposite direction to the direction of rotation of the drill bit. One result of bit whirl is that some cutters may temporarily move in the reverse direction relative to the formation and this can result in damage to the cutting elements.
It is believed that the stability of such a drill bit, and its ability to resist vibration, may be enhanced by increasing the area of the bearing surfaces on the gauge region which engage the wall of the borehole. In the prior art designs, however, the area of engagement can only be increased by increasing the length and/or width of the bearing surfaces on the kickers. It may be undesirable to increase the length of the bearing surfaces since this may lead to difficulties in steering the bit in steerable drilling systems. Similarly, increasing the circumferential width of the bearing surfaces necessarily reduces the width of the junk slots between the bearing surfaces, and this may lead to less than optimum hydraulic flow of drilling fluid along the channels and over the cutters, or it may lead to blockage of the junk slots and channels by debris.
The present invention relates to a number of improvements to drill bits of this type.